To fault find a system you need to know EXACTLY how it works
HOW EXACTLY DOES A PLC WORK?
Am I getting the input to the PLC?
The Led on the output card means i am getting voltage out right? does it?
What exactly happens in between? ,theres more than just a program in the CPU
How exactly does it scan the program?
What is this Watchdog Timer? Is it that important?
Can I use the same output twice? That's bad programming isn't it?
A PLC is a logic controller, so use a logical approach to fault find it.
What are the 8 simple test points to check?
The PLC is in RUN, that means theres a program right? does it?
FORCING a bit and toggling a bit is pretty much the same yeah? depends on which PLC
Then you need to Know the specifics
HOW DO I DO THE FOLLOWING?
(some straight forward some not so)
- How do I check power is ON and PLC is in right mode (RUN or Program)
- Check for a fault condition
- Establish a link between PC and PLC (can be a major issue nowadays)
- If comms problem check interface
- Create a blank project and take a backup (just in case I mess up)
- Open project for PLC and go OnLine
- Interrogate Diagnostics
- Identify if it is a hardware or software fault?
- Change the battery
- Change modules if necessary, (with spares and without spares)
- Identify if it is a PLC or Comms fault
- Check all settings against a template etc.
- Check Hardware ( What voltage should be where)
- Clear Memory and Download program
- Check software against latest copy
- Monitor program effectively
- Searching for specific operands and instructions
- Changing timer, counter values On Line
- Making minor mods Off Line
- Check or create a monitor table to establish parameter status
- Force a parameter if required
- Call up reference data to assist with software diagnostics
- Display Documentation (Symbols, Comments)
- Printing Cross Reference / Program Listings etc.
Background information required
Understanding of the following:
- Number formats, bits, bytes, words, double words
- Binary, HEX,octal,floating point, integer
- Data types and parameter types, Bool etc.
- On Line and Off Line modes
- Basic Instructions, contacts, Set, reset etc.
- Timers, Counters
- Comparators, Maths
- How to make minor mods
- Altering values in a monitor table
- Using Device Monitor function
- Using Force function
- Back tracking through a program to establish where power flow stops
- Reset procedure
- Fault finding tips